The present invention relates generally to synthetic array radar (SAR), and more particularly, to a method and apparatus, commonly know as autofocus, that automatically corrects SAR focus errors.
Synthetic array radar resolution and the utility of its imagery depend upon accurate focus correction. Conventional autofocus techniques include phase comparison autofocus, map drift autofocus and phase difference autofocus, all developed by Hughes Aircraft Company. These methods estimate coefficients of a polynomial that best characterizes the focus error in some sense. A polynomial representation is not always adequate, however, when the focus error history is highly non-linear and can not be accurately denoted by a polynomial of a lower order.
Nonconventional autofocus techniques that are capable of correcting a phase error of a very high order include a prominent scatter technique developed by the Environmental Research Institute of Michigan, a single target discrete autofocus developed by the Hughes Aircraft Company, and a phase derivative autofocus developed by Sandia National Laboratories. The prominent scatterer technique required a presence of an isolated point scatterer. The single target discrete autofocus technique did not require an isolated target. It also eliminated a need to unwrap phase data to create a continuous phase function. But, both the prominent scatterer and the single target discrete autofocus techniques were susceptible to a target dependent phase error since a phase correction signal was obtained from a single target. The single target discrete autofocus technique is described in U.S. Pat. No. 5,043,734, for "Discrete Autofocus for Ultra-High Resolution Synthetic Aperture Radar Imaging," assigned to the assignee of the present invention. An attempt to reduce target-dependent phase error did not succeed since a method could not be found to average phase data properly. This short fall was resolved by the Sandia's phase derivative autofocus by computing phase derivatives from many targets and averaging derivatives to eliminate target dependent phase errors. Averaged phase derivatives are then used to construct a phase correction signal with target-dependent errors substantially reduced. The Sandia system is described in a paper entitled, "A New Phase Correction Method for Synthetic Aperture Radar", by P. H. Eichel, et al., IEEE Conference on Digital Signal Processing, September 1988. The Sandia system is considered to be the closest related system to the present invention. Based upon the contents of this paper, however, it is believed that the Sandia's system does not optimally weight data from various scatterers, uses unnecessary iterations, and the window length design rational is undefined.
Consequently, there is a need for a nonconventional autofocus technique that optimally weighs data from various scatterers, eliminates unnecessary iterations, and selects window length optimally and adaptively in an efficient, cost effective manner.